Nothing wrong with those measurements
With the pot set to 4.7K the circuit is trying to get an output voltage of around 26V which it can't do with a 16V input. Set the pot around 2k and you should get about 12V

Nothing wrong with those measurements
With the pot set to 4.7K the circuit is trying to get an output voltage of around 26V which it can't do with a 16V input. Set the pot around 2k and you should get about 12V

Click to expand...

Hi Albert,

Thanks a lot for your fast reply.

So, Vref must be 1,25 V with Pot 4.7KOhm set to 0 KOhm, and lower like 0,7V with increasing Pot value ????

So, Vref must be 1,25 V with Pot 4.7KOhm set to 0 KOhm, and lower like 0,7V with increasing Pot value ????

I'm wrong or right?

Click to expand...

Wrong and right!. The pot will vary the output voltage and the ref voltage will remain fixed at 1.25V until the requested output voltage is getting close to the input voltage. At that point the regulator will no longer be able to regulate, the output will increase no further and the ref voltage will fall.

Your circuit is not ideal.
The transistors will amplify the changes on the output of the regulator.
Normaly they use a PNP transistor for boosting the LM317.
In the picture below (taken from the german magazine FUNKAMATEUR 3/2016) , you will see what the PNP transistors does in the circuit:

If you want to use the power NPN 2N3055 in bertus's circuit (which is indeed the proper way to boost the current of an LM317) you can use it with a small PNP (such as a 2N2907) in aSziklai pair(sort of a complementary Darlington) to emulate the PNP power transistor.

So, voltage on adj pin = 0V and voltage across R1 is zero, therefore the voltage on the 2N3055 base connections is zero. But the output voltage is 34V? So D2 or one of the 2N3055 transistors is short circuit.
(all of this assumes everything wired as per schematic)

As the pot is connected between adj and ground then we have 34V across the pot. What was the setting of the pot at that time?
Set the pot around mid-way. Measure all three pins of the LM317 and the output voltage of the supply (i.e. voltage across C3).

As the pot is connected between adj and ground then we have 34V across the pot. What was the setting of the pot at that time?
Set the pot around mid-way. Measure all three pins of the LM317 and the output voltage of the supply (i.e. voltage across C3).